It is well known that an increasingly important consideration in the production and use of integrated circuits (ICs) is the package in which the IC resides. The module or casing in which the IC is packaged is a prominent factor in the ultimate cost, performance and lifetime of the IC. An important consideration as the circuits become more dense is that the number of leads to the package and connections from the leads to the integrated circuit pads increases; thus increasing the complexity of construction and adding to the cost of the end product, not just in terms of increased and more expensive materials, but also increased production costs. A package which has satisfactorily addressed the need of a large number of interconnections is the pin grid array (PGA) where a plurality of pins oriented normal to a relatively flat package body gives a "bed of nails" appearance. PGAs have proven popular when hundreds of connections must be made.
However, another factor affecting the design of IC packages is the advent of surface mount technology, whereby space is conserved on the printed circuit boards (PCBs) by mounting the packages directly on the conductive patterns of the circuit board, rather than by extending the leads through holes in the board. This technology is an additional influence in making the packages smaller, and making it more difficult to design a small package that will also contain a large number of leads. An additional problem with surface mount technology is that in addition to aligning the leads properly laterally, which is more difficult as the lead number increases, the leads must also be coplanar, that is, in the same plane or aligned in the vertical dimension so that the bonds to the surface bonding lands of the PCB pattern may be made without gaps or shorts.
Thus, a continuing purpose in the art of providing packages for electronic components, such as integrated circuits, is a package design that will address these multiple goals satisfactorily in an arrangement that can be reliably manufactured at the lowest cost. The lowest cost packages are those which have plastic bodies which can be molded from thermoplastic and thermoset materials.
Addressing all of these goals has proven difficult. It has been hard to surface mount a package containing large numbers of leads; PGAs must either be mounted through holes in the PCB or in a carrier that is in turn surface mounted. Additionally, PGAs are generally made of expensive ceramic materials, rather than the less expensive plastic compounds. Further, a large PGA with a relatively rigid structure having 500 leads or more would be subject to mechanical displacement over the package area as the PCB, and the package in turn, are mechanically flexed or subjected to non-uniform heating that would cause one flat area of the package to expand at a rate different than an adjacent area of the package. Such stresses could cause bonds to come loose causing shorts or could crack the package body undesirably providing entry to moisture and other contaminants. These stresses are aggravated when the package body is fabricated from a material that is considerably different from the PCB substrate, for example, where the package is ceramic and the board is plastic with different rates of thermal expansion. Thus, the issues of very large pin counts, thermal dissipation, surface mounting and low cost are intertwined.